Although I followed various other articles on the internet that suggested various good fixes mine was caused by something different. I include the other articles for completeness.
I had 1 template set to the default template, but I had assigned another template to all pages which caused me this problem.
I think my situation was unusual so I include the other information to help anyone else who arrives here.
Option #1. Change Error Reporting to Maximum
set error reporting to maximum
Try to log into your Joomla administrator area. If you can get in, go
to Global Configuration > Server > Error Reporting and set it to
Maximum. You can find a more detailed tutorial for this here. Now
check the front of your site to see if any errors are displaying. If
you can't access your site's administrator area, set this in your
configuration.php file:
public $error_reporting = 'development';
Option #2. Edit Files in FTP
Log in to your Joomla site files via FTP. Find your index.php file
located in the root of your site. Open the index.php and add this code
directly after the very first line of the file:
ini_set('display_errors', TRUE); error_reporting(E_ALL);
Option #3. Edit PHP.ini file
The PHP.ini file is a vitally important file that controls many of
your server's settings. If you don't know where it is, use this
tutorial to find it.
We recommend that you change these two settings:
Set Display error to On:
display_errors = On
Set error reporting:
error_reporting = E_ALL & ~E_NOTICE & ~E_STRICT
Option #4. Check Your Server Log
Check your server error log for any indication of what is causing the
issue.
Check your Joomla server log
If you are not sure how to access this, you can contact your hosting
company.
If you are on localhost, you can usually find it in a directory called
"logs" within the installation folder of your local server. You've
found the error message. Now what?
When you have discovered what is causing the error message, it's time
to analyze it. You will hopefully see errors that resemble those in
the image below.
Even if you don't understand the message, you know which extension is
causing problems
In these examples, the errors come from sh404SEF and from Xmap. Even
if you don't understand the message, you know which extension is
causing problems.
If the name of an extension can't be found, you might see an error
message like the ones below. This may mean that there's a problem with
the Joomla core files.
What can you do now that you've identified the troublesome extension?
What can you do now that you've identified the troublesome extension?
Problems with an extension? Disable the extension
This tutorial shows how to disable a plugin, but applies to other
types of extensions also.
The quickest and easiest solution is to log in to your site via FTP.
If the troublesome extension is a component, go to the /components/
folder and change the name of the extension folder.
For example, change /com_sh404SEF/ to /com_sh404SEF_disabled/.
You may also need to change the directory name within the component in
the /administrator/components/ directory as well. Problems with the
Joomla core? Reupload the core files
If you didn't install any extensions and you think it may be caused by
the core Joomla files, try reinstalling all of Joomla core files.
You can do this by downloading the latest version found on joomla.org
and overwriting all the directories to your servers FTP.
If you have access to the administrator area, you can also try Step 5
from this tutorial.
